1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a process that uses a fluoropolymer film as a method of defining the spacing between adjacent printed ink structures.
2. Description of the Related Art
Metal lines are commonly formed in printed electronic applications by inkjet printing of metal nanoparticle or metal precursor inks onto a substrate. The line width and line shape are dictated by the printed volume of ink and the interaction of the ink with the surface of the substrate. It is important to tailor the surface energy of the substrate for a specific ink to achieve the desired line characteristics. In order to create a thin film transistor (TFT) by inkjet printing, two inkjet printed metal layers are typically required: a gate metal and source/drain metal. The source/drain print characteristics are of particular interest because they typically determine the transistor channel length (L)
The accuracy of inkjet drop placement limits how far down the channel lengths can reasonably be scaled while preserving device yield. Misplaced drops can cause the printed source and drain lines to merge, for example, and cause an electrical short. There are many factors that affect drop placement. Some of these factors are the hardware limitations of the printer. A couple examples of hardware limitations are the inherent accuracy of the substrate stage movement or movement of the inkjet cartridge. Typically these positional limitations are on the order of 5 to 10 microns (μm) for commercially available printers today. Repeatable printing of lines with spacings below these values is unlikely without other process improvements.
FIG. 1 is a plan view of a process that takes advantage of the coffee staining behavior of printed fluoropolymer (prior art). As described by Kawase Takeo in U.S. Pat. No. 6,838,361, fluoropolymer 14 is printed and dry etched to define a fluoropolymer ridge in the shape of a racetrack. This ridge is used to separate a printed ink line 20 on either side of the fluoropolymer ridge. In this way a source and drain metal line are printed with a gap equal to the width of the fluoropolymer ridge. One problem with this approach is the reliance upon the drop placement accuracy of the inkjet printer to determine the relative amounts of ink that fall on either side of the fluoropolymer ridge. If the printed metal line is not centered on the ridge, the differences in ink volume are asymmetrically distributed on either side of the ridge. This has device consequences such as source/drain lines with different widths, electrical conductance, trace height, and surface roughness to name a few.
Another problem with the above-described method is the requirement of a dry etch to remove the residual fluoropolymer material from the center of the racetrack structure. It is not clear whether the surface energy of the substrate inside and outside of the racetrack structure are equivalent after this dry etch process. If different surface energy characteristics exist in these two areas the metal precursor ink spreads in different ways causing again the source and drain lines of the TFT to have different characteristics.
It would be advantageous if the placement and shape of printed ink structures could be more precisely controlled.
It would be advantageous if the placement and shape of printed ink structure was less dependent upon printer drop placement tolerances.